1. Field of the Invention
The present invention relates to heat exchanger evaporators, especially to a counterflow evaporator optimized for zeotropic refrigerants having significant glide characteristics. In particular, the invention relates to a shell and tube type evaporator, where the refrigerant flows through the tubes and evaporates, while a fluid flows through the shell and is cooled by the evaporating refrigerant. The evaporator is a component of a refrigeration system which can be used for cooling large quantities of water.
2. Description of Related Art
Refrigeration systems of the type used to cool large quantities of water typically include a heat exchanger evaporator having two separated passageways. One passageway carries refrigerant, and another carries the fluid to be cooled, usually water. As the refrigerant travels through the evaporator, it absorbs heat from the fluid and changes from a liquid to a vapor phase. After exiting the evaporator, the refrigerant proceeds to a compressor, then a condenser, then an expansion valve, and back to the evaporator, repeating the refrigeration cycle. The fluid to be cooled passes through the evaporator in a separate fluid channel and is cooled by the evaporation of the refrigerant. The fluid can then be routed to a cooling system for cooling the spaces to be conditioned, or it can be used for other refrigeration purposes.
One method of increasing the efficiency of heat exchanger evaporators in general, especially those of shell and tube type, is to vary the number and the dimensions of the tubes carrying the refrigerant. This approach, however, results in a prohibitive cost increase.
Another approach used to increase the efficiency of heat exchangers in general has been to install rods in heat exchanger tubes, to form annular passages within which a fluid flows. Applications of this approach are disclosed in U.S. Pat. No. 1,303,107 to Oderman; U.S. Pat. No. 3,749,155 to Buffiere; and U.S. Pat. No. 5,454,429 to Neurauter. This approach increases heat transfer through the outer wall of the annulus by increasing refrigerant flow rate near the wall. However, this approach often has drawbacks. For example, galvanic corrosion between metal parts made of different metals can cause premature failures of the heat exchanger and require excessive maintenance and repairs. When the rods are used within the tube passages, the energy of the flow can cause the rods to vibrate. The acoustic energy developed by the interaction between the flow and the rods in the tubes can damage the structure of the evaporator over time. In some application, this approach causes a high pressure drop across the tube, thereby reducing the efficiency of the refrigeration cycle. Moreover, applications of this approach often have increased the costs of the resultant heat exchanger substantially, because of the material costs of the rod and the material and labor costs associated with installing and holding the rod within the tube.
Recently, certain regulatory bodies have placed restrictions on the types of refrigerants that can be used in certain refrigeration applications. In view of these restrictions, along with the above limitations on existing evaporator designs, there continues to exist a need for an improved evaporator for refrigerants.